As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2006.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
Multilayer tubes consisting of co-extruded polyethylene or polypropylene tubes with a metal core have found many applications in the building industry, for example as sanitary pipes, heating pipes, gas distribution pipes etc., as well as in other industries such as the pharmaceutical, chemical or food industries. Compared to plastic tubes, multilayer tubes having an aluminium alloy core layer are impermeable to oxygen, have higher strength and heat resistance, as well as a low expansion coefficient. Such multilayer tubes are easy to install since they can be bent and cut to the desired length on site. They are also preferred to tubes made of metal only, since the plastic inner and outer layers improve the corrosion resistance and serve as noise barrier. They also save weight and have a better formability.
Such multilayer tubes have an outer plastic layer, an inner plastic layer and an aluminium core layer for mechanical strength and long time performance. The inner and outer plastic layers are generally bonded to the aluminium core by means of an outer and inner adhesive layer, respectively.
The most critical property of multilayer tubes in heating and sanitary applications is the resistance to constant and sustained internal pressure at increased temperatures. This is generally tested by the standardized internal pressure tests ASTM-F1281, ASTM-F1282 or DVGW-W542 in which the tube is kept at 30 bar and 95° C. until the tube fails. From the time until failure, one may extrapolate the expected lifetime of the tube under the conditions encountered, for example, in the sanitary system of a building, i.e. a temperature of 70° C. at 20 bar. A high resistance against sustained internal pressure is especially important if the metal core layer is to be kept as thin as possible, and for tubes with larger diameters.
Aluminium alloys currently used as core layer in multilayer tubing are AA3003 and AA3005-series aluminium alloys.
The AA3003 aluminium alloy has a chemical composition, in wt. %, of:
Si<0.6Fe<0.7Cu0.05 to 0.20Mn1.0 to 1.5Zn<0.10,                impurities <0.05 each, total <0.15,        balance aluminium.This aluminium alloy has previously been used in multilayer tubes because it is relatively inexpensive and has sufficient strength. However, it does not perform well under the sustained pressure tests.        
EP-1323839-A1 discloses an aluminium brazing alloy suitable as fin stock material, having a composition, in wt. %, of: Si 0.7 to 1.2, Mn 0.7 to 1.2, Mg<0.2, Fe<0.8, Zn<3.0, Ni<1.5, Cu<0.5, optionally one or more elements selected from the group comprising (Ti<0.20, In<0.20, Zr<0.25, V<0.25, Sn<0.25, Cr<0.25), impurities <0.05 each, total <0.15, balance aluminium. This composition was chosen because of its improved post-braze 0.2% yield strength in comparison to conventional brazing alloys. The aluminium alloy also has a good corrosion resistance and an improved tolerance for impurity elements. The resistance of the alloy to sustained pressure, as required for welded multilayer tubes, has not been tested or considered.
A need therefore exists for an aluminium alloy for use in multilayer tubes that has improved resistance to constant and subtained internal pressure, ideally at elevanted temperature of for example 95° C.